1. Field of the Invention
This invention relates to a method of migrating a melt of a metal through a solid body of semiconductor material by thermal gradient zone melting (TGZM) and, in particular, to the uniform initiation of migration by enhancing the penetration of melts into the material at the surface when fine liquid lines or small diameter liquid droplets are being migrated.
2. Description of the Prior Art
W. G. Pfann in U.S. Pat. Nos. 2,739,088 and 2,813,048 describes methods of migrating melts of metal through particular regions of a solid body of semiconductor material by thermal gradient zone melting. However, molten line and droplet stability resulted in the breakup of the migrating lines and droplets and consequently acceptable semiconductor devices were not always obtainable.
Recently, we discovered that preferred planar orientation of the surfaces of the body of semiconductor material, migration axis and line orientation axis relationship were also a necessity to migrate liquid metal wires and/or droplets through the solid body. (See U.S. Pat. Nos. 3,899,362 and 3,904,442) for example. These improvements in TGZM resulted in commercialization of the process. However, as the width of the lines being migrated became smaller, the penetration of fine liquid lines of less than 2 mil in width, and preferably 1 mil in width, and small liquid droplets, less than 6 mils in diameter, from the surface of a wafer or body of semiconductor material has been difficult to achieve repeatedly, on a commercial basis, by a thermal gradient zone. Although a thermal gradient is strong enough to cause migration of the small liquid zones once they are formed in the bulk of semiconductor material, the thermal gradient force is not powerful enough to overcome the surface tension forces holding fine liquid zones, or wires, on the surface of a body, or wafer. Further improvements to the TGZM processing techniques included alloying the deposited metal to the surface (U.S. Pat. No. 3,897,277) and sintering of the same (U.S. Pat. No. 4,006,040). The problem still persists as one attempts to migrate fine lines and droplets on a commercial basis. As a result, TGZM to date has been limited to line and droplet dimensions typical of solid-state power devices and has not had any commerical impact on integrated-circuit type devices which require a much finer size doped region.
Therefore, it is an object of this invention to provide a new and improved method to migrate fine molten lines and droplets of metal through a solid body of semiconductor material by thermal gradient zone melting (TGZM) processing which overcomes the deficiencies of the prior art.
Another object of this invention is to provide a new and improved method for initiating the penetration of fine molten metal wires and droplets into the surface region of a solid body for migration therethrough by TGZM by providing an overcoating of the semiconductor material on the metal to be migrated by TGZM.
A further object of this invention is to provide a new and improved method of initiating fine metal molten wires and droplets into the surface region of a solid body for migration therethrough by TGZM by providing a melt which is formed from a codeposited material consisting of at least one dopant material and material which is of the same material of the solid body.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.